1. Field of the Invention
In the field of papermaking, for example, aqueous suspensions are produced which contain contaminant particles of various types and sizes. These contaminant particles are often hydrophobic and sticky. They may lead to deposits in machinery which have to be removed during expensive down-time periods. Such deposits consist, for example, of poly(vinyl) acetate, polystyrene, polyacrylate, wax, paraffin wax, natural rubber or other substances which often originate from recycled adhesive tapes, hot-melt adhesive bridges, self-sealing envelopes or from coating waste or bituminized or waxed paper. To prevent such deposits, one option is to attempt to either filter out the contaminants or to precipitate them; alternatively, the pulp is admixed with chemical additives which bind the contaminant particles in finely dispersed form to the wood fibers in the pulp and thus prevent deposition in the machines. The quality and the economic efficiency of papermaking in this context depend on the choice of additive and how much of it is added. If the type, amount and size of the contaminant particles fluctuate, this has to be reflected in changes in the type and amount of the additive added. This in turn is only possible if the type, amount and size of the contaminant particles are known as accurately as possible. In this context it is particularly important to know the size distribution of the contaminant particles.
2. The Related Art
Conventional test methods, for example X-ray microanalysis, infrared spectrophotometry or gel permeation chromatography, as reviewed in R. Wilken and J. Strauss, "Grundlegende Untersuchungen uber klebende Verunreinigungen im wiederverwendeten Altpapier" Fundamental studies on sticky contaminants in reused wastepaper!, Mitteilungen aus dem Papiertechnischen Institut der Papiertechnischen Stiftung, Vol. 11-12 (1984), p. 292 et seq. can be used to determine the type of contaminant particles, ie. their chemical composition, in the laboratory. Qualitative statements on the concentration and particle size distribution are also possible. All these methods have the drawback, however, that they are relatively time-consuming and laborious and are therefore unsuitable for direct monitoring of changes in contaminant and of the effect which additives have on the binding of the contaminants to the pulp during the production cycle.
A different method for determining the particle size distribution of contaminant particles is described in T. Krohl, P. Lorencak, A. Gierulski, H. Eipel and D. Horn, "A new laser-optical method for counting colloidally dispersed pitch", Nordic Pulp and Paper Research Journal, Vol. 9 (1994), No. 1, p. 26 et seq. In this method, contaminant particles are singled by hydrodynamic focusing in a cell and are stained with a fluorescent dye. Laser light is then radiated into the sample containing the singled contaminant particles, and fluorescent light emitted by these is recorded. The particle size distribution can then be inferred from the intensity of the fluorescence signals. This method, though, will not provide a sufficiently precise particle size distribution unless the sample either contains only one particle type or contains a plurality of particle types which, however, have approximately equal stainability for the fluorescent dye used and comparable quantum yield. Since these prerequisites are rarely met in practice, the fluorescence-optical measuring method does not provide a procedure reliable in practice for determining the particle size distribution in a sample containing a number of different-type particles. A further drawback is that it is not possible to distinguish between a plurality of different particle types. Consequently, the additive cannot, in terms of type and quantity, be tailored to the conditions applying in each particular case.
Problems similar to those in papermaking also exist in many other industrial fields, in particular with recycling processes, where contaminants or alternatively valuable substances have to be identified as precisely as possible, so that their treatment can be defined appropriately.